1. (FIELD OF THE INVENTION)
The present invention relates to a power steering system which can make it easy to operate the steering wheel of an automotive vehicle. More specifically, the invention relates to a power steering system including a reaction means for applying a reaction force to the steering wheel of the vehicle so as to react steering operation in accordance with vehicle driving conditions.
2. (DESCRIPTION OF THE PRIOR ART)
A power steering system is described in the Japanese Patent First Publication (Tokkai) Showa No. 49-120330. This power steering system includes means for reducing the manual steering force and a reaction means for applying a reaction force to the steering wheel of a vehicle. The reaction means comprises two reaction chambers. The means for reducing the manual steering force comprises a control valve and a spool valve housed in the control valve, a power cylinder which can be in communication with the control valve, and an actuating pin mounted on the spool valve. When the driver operates the steering wheel, the spool valve is caused to move, by means of the actuating pin, in one of axial directions. As a result, a port formed on the control valve is in communication with an annular groove provided on the spool valve so that high-pressure working fluid is supplied from a working-fluid tank to one of chambers of the control valve. As a result, an assisting force is generated to reduce the manual steering force which must be applied by the driver. In addition, a part of the working fluid is supplied to one of the reaction chambers to restrict displacememt of the spool valve. On the other hand, the working fluid in the other reaction chamber is drained to the tank by means of a variable orifice. In the case of steering operations performed while the vehicle is stopped, the variable orifice opens fully by means of a spring to decrease the fluid pressure in the reaction chambers. Conversely, in the case of steering operations performed when the vehicle is traveling, the opening of the variable orifice is decreasing as the vehicle speed is increasing to increase the fluid pressure in the reaction chambers.
However, in the aforementioned power steering system, the quantity of the working fluid drained to the tank is increased as the pressure in the system increases and an assisting force enough to sufficiently decrease the manual steering force can not be generated. In order to compensate sufficient quantity of working fluid supplied to the power cylinder, the capacity of the pump must be enlarged. Therefore, there is a disadvantage in that the power steering system becomes large, heavy and expensive.